There are several instances of medically useful devices which are mechanically slender and flexible and are also inserted through the skin.
For example, sensors facilitate the sensing of certain conditions within a patient. Electrochemical sensors are commonly used to monitor blood glucose levels in the management of diabetes. In one scheme, an electrochemical sensor incorporating an enzyme is fabricated onto a small diameter wire. A second reference electrode is also fabricated around the wire near the sensing electrode. The sensor assembly is inserted through the skin so that it is surrounded by interstitial fluid. A portion of the sensor assembly exits the skin, remaining outside the body, where electrical connections to the sensing electrode and reference electrode may be made. A suitable electronic measuring device outside the body may be used to measure electrical current from the sensor for recording and display of a glucose value. These types of devices are described, for example, in U.S. Pat. No. 5,965,380 to Heller et al. and U.S. Pat. No. 5,165,407 to Ward et al.
In addition to electrochemical glucose sensors, a number of other electrochemical sensors have been developed to measure the chemistry of blood or other body fluids or materials. Electrochemical sensors generally make use of one or more electrochemical processes and electrical signals to measure a parameter. Other types of sensors include those which use optical techniques to perform a measurement.
In other applications, a cannula and sensor combination device is inserted through the skin to allow insulin to be introduced into the body as part of an artificial pancreas system. In these applications, a slender (small cross-section) and flexible device offers several advantages over a larger and more rigid device. Patient comfort is increased, especially during long-term insertion, and trauma at the entry site is reduced. A flexible device also is able to adjust to movement of the skin during physical activity, increasing patient comfort. In many cases these devices will remain inserted in the body for 5 to 7 days.
Although the slender and flexible nature of these devices increases patient comfort, these devices are difficult to insert through the skin. Unlike a typical hypodermic needle, these devices are too fragile and flexible to be simply pushed through the skin surface using normal force and speed. When the tip of such a device is forced against the skin, the device will bend and collapse with much less force than would be required to achieve skin penetration. Although in some cases the tip of the device may be sharpened to ease penetration, this approach is not typically adequate to assure penetration, and some devices such as tubing-based devices are not appropriate for sharpening. Also, the sharpening process adds to production cost and complexity.
As will be understood by those skilled in the art, human skin possesses biomechanical properties influenced by a relatively impenetrable outer layer, the stratum corneum, and inner layers which are more easily penetrated. These biomechanical properties cause penetration of the skin surface to present the primary challenge in introducing a relatively fragile slender, flexible device into the skin.
Current art provides several approaches for insertion of such slender flexible devices through the skin. In one case, the device is placed coaxially inside a hollow tube with a sharpened end, such as a hypodermic needle or trocar. The needle is inserted through the skin with the device inside. As a second step, the needle is withdrawn, leaving the device behind, passing through the skin into the body. See, for example, U.S. Pat. No. 6,695,860 to Ward et al. The insertion process may be painful, due to the large diameter needle, and a larger opening is made in the skin than required for passing the device alone, increasing trauma and the possibility of infection.
In a variation of this approach, the functions of the device are incorporated into a thin needle which must stay inserted into the skin. The needle provides additional mechanical strength and a sharpened point to assist in piercing the skin. However, due to its larger size and rigidity, this approach also contributes to patient discomfort for the duration of the insertion. See, for example, U.S. Pat. No. 6,501,976.
In addition, the presence of a rigid needle places mechanical constraints on the size and shape of the device housing that is attached to the surface of the skin where the device exits the skin. The needle also must be treated as a biohazard “sharp” since it is capable of transmitting disease if it should accidentally puncture the skin of another individual after being used in device insertion.